Adjustment for hydrocephalus shunt valve

ABSTRACT

An implantable medical device is disclosed that includes a valve seat and a valve member movable with respect to the valve seat. An adjustment circuit assembly includes a resistive element that is coupled to the valve member and operable to position the valve member relative to the valve seat so as to alter the pressure setting in response to a current applied to the resistive element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/015,174, filed Jan. 27, 2011, now U.S. Pat. No. 8,298,168, andentitled “Adjustment for Hydrocephalus Shunt Valve.”

BACKGROUND

1. Technical Field

This disclosure relates generally to surgically implanted physiologicalshunt systems and related flow control devices. More particularly, thepresent disclosure relates to a position indicator and adjustment toolfor such shunt systems having variable pressure settings for the one-wayflow control valves controlling the flow of Cerebral Spinal Fluid (CSF)out of a brain ventricle and preventing backflow of fluid into the brainventricle.

2. Description of Related Art

A typical adult has a total of about 120-150 cubic centimeters (cc) ofCSF with about 40 cc in ventricles in the brain. A typical adult alsoproduces about 400-500 cc/day of CSF, all of which is reabsorbed intothe blood stream on a continuous basis.

Sometimes, the brain produces excess CSF or there can be a blockage ofthe normal CSF pathways and or absorption sites resulting in a conditionknown as hydrocephalus. Hydrocephalus is a condition of excessiveaccumulation of CSF in the ventricles or brain tissue. Hydrocephalus canresult from genetic conditions, congenital defects infection, cancer,hemorrhage trauma to the brain or as a person ages.

Excessive accumulation of CSF, due to hydrocephalus or other causes,manifests itself as increased pressure within the brain. Whatever thecause, over time, this increased CSF pressure causes damage to the braintissue. It has been found that relieving the CSF pressure istherapeutically beneficial. This relief is usually performed by drainingCSF from the ventricles.

Patients with hydrocephalus normally require, at least over some timeperiod, continuous drainage of excess CSF to maintain normal CSFpressure in the brain. Excessive CSF accumulated in the ventricles ofthe brain is typically drained away from the brain using a shunt system.

Where hydrocephalus is a chronic condition, the shunt system typicallydrains the CSF into the patient's peritoneal cavity or into thepatient's vascular system. Such shunt systems typically have a catheterimplanted in the ventricle of the brain. The catheter is connected to afluid control device which is in turn connected to a catheter whichempties in to the patient's peritoneal cavity or into the patient'svascular system. An example of a fluid control device is shown in U.S.Pat. No. 5,637,083 issued to William J. Bertrand and David A. Watson onJun. 10, 1997 entitled “Implantable Adjustable Fluid Flow ControlValve”, the teaching of which is incorporated herein in its entirety byreference. Current fluid control devices include an inlet connector, anoutlet connector and a valve positioned between the inlet connector andthe outlet connector. The valve includes a mechanism to control fluidflow through the valve. In some instances, the mechanism includes amagnet embedded within the valve. Rotating a rotor or otherwise shiftingof the rotor position changes the internal configuration of themechanism. Changing the internal configuration of the mechanism producesa variety of pressure or flow characteristics for the valve. As theinternal configuration of the valve changes, the pressure or flowcharacteristics of the valve change.

In use, the valve is subcutaneously placed on the patient's skull. Thecatheter going to the patient's ventricle is attached to the inletconnector. The catheter going to the patient's peritoneal cavity orvascular system is attached to the outlet connector. In this way, adirection of flow is established from the inlet connector through thevalve to the outlet connector. Changing the internal configuration ofthe mechanism by coupling the external magnet to the internal magnet androtating the external magnet effects a movement internal to the shuntand produces a variety of pressure or flow characteristics through thevalve.

It is desirable to have a number of different settings in order toachieve different pressure and/or flow characteristics of the valve. Onecomplication with current adjustable valves is that once implanted, itis difficult to determine the setting of the valve and/or adjust thesetting of the valve. Having more settings for the valve only makesdetermining and/or adjusting the valve setting more difficult. With someadjustable valves, x-ray images are used to determine the current stateor post adjustment state of the valve. By requiring an x-ray, it is timeconsuming and costly to determine and adjust the valve setting, as wellas not being in the best interest of the patient due to x-ray exposureissues.

Another complication with current adjustable valves is compatibilitywith magnetic resonance imaging (MRI) procedures. As many currentadjustable valves utilize magnets for adjusting and/or determining avalve setting, their function can be disrupted due to interaction ofmagnetic components in the valve with the applied magnetic field createdduring the MRI procedure. In particular, the valve setting can bealtered to a random, undesirable setting. If the valve setting is notreturned to the desired setting after the MRI procedure, this situationcan be extremely harmful to a patient. As such, the valve setting needsto be immediately reset to the desired setting upon conclusion of theMRI procedure. In any event, improvement of valves for the treatment ofhydrocephalus can provide great benefit.

SUMMARY

Concepts presented herein relate to determining and/or adjusting apressure setting for an implantable medical device. In one embodiment,the device includes a valve seat and a valve member interfacing with thevalve seat to establish a pressure setting indicative of a pressurewhere fluid will flow through the valve seat. An adjustment circuitassembly includes a resistive element that is coupled to the valvemember and operable to adjust force on the valve member relative to thevalve seat so as to alter the pressure setting in response to a currentapplied to the resistive element.

In another aspect, an implantable flow control device includes a valvedefining a plurality of pressure settings. A connector assembly iscoupled to the valve and movable with respect to the valve to adjust apressure setting for the valve. An adjustment circuit assembly includesa setting coil, adjustment capacitor and resistive element electricallyconnected to one another. The resistive element is coupled to theconnector assembly. A reading circuit assembly includes an antenna coil,a reading capacitor and a sensing coil electrically connected to oneanother. The reading circuit assembly also includes a moveable membercoupled to the connector assembly and moveable with respect to thesensing coil. Current induced in the setting coil causes the resistiveelement to move the connector assembly relative to the valve. Currentinduced at the proper frequency in the antenna coil generates a signalindicative of the pressure setting.

In another aspect, a method of controlling flow of fluid in animplantable device includes providing a valve in the device adjustableto a plurality of pressure settings. An oscillating electromagneticfield resonant with an adjustment circuit assembly coupled with thevalve is received. Current is induced in a resistive element of theadjustment circuit assembly and a pressure setting of the device isadjusted based on the current in the resistive element.

In yet a further aspect, a method of operating an implantable flowdevice includes providing a valve adjustable to a plurality of pressuresettings and coupling a connector assembly to the valve. The connectorassembly is movable with respect to the valve to adjust a pressuresetting for the valve. A reading circuit assembly is further providedthat includes an antenna coil, a sensing coil and a member movable withrespect to the sensing coil. The member is connected to the connectorassembly such that a resonant frequency of the reading circuit assemblyis altered as a function of a position of the connector assembly withrespect to the valve.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of an adjustable shunt system.

FIG. 2 is a schematic block diagram of an adjustment circuit assemblypositioned in a flow control device.

FIG. 3 is a schematic block diagram of a reading circuit assemblypositioned in a flow control device.

FIG. 4 is a perspective view of a first embodiment of a flow controldevice.

FIG. 5 is a cross-sectional view of the flow control device illustratedin FIG. 4.

FIG. 6 is a perspective view of a second embodiment of a flow controldevice.

FIG. 7 is a cross-sectional view of the flow control device of FIG. 6.

DETAILED DESCRIPTION

FIG. 1 is a schematic block diagram of an adjustable shunt system 10including an implantable flow control device 12 (e.g., a shunt) and anelectronic valve reader and adjustment tool 14. In general, device 12can be implanted in a patient to regulate flow of fluids (e.g., CSFdiscussed above) within the patient based on a pressure setting (alsoknown as a valve setting) for the device 12. Tool 14, in turn, can be ahandheld mechanism configured to subcutaneously read and adjust thepressure setting of the device 12 when positioned proximate thereto. Assuch, tool 14 is a non-contact device capable of reading and adjustingdevice 12 while being located outside the body of a patient. Inparticular, the tool 14 can create an oscillating electromagnetic fieldthat is received by device 12. The field can cause device 12 to adjustthe pressure setting and provide feedback indicative of a pressuresetting as will be discussed below.

The device 12 includes a valve 16, an adjustment circuit assembly 18, areading circuit assembly 20 and a connector assembly 22 coupling thevalve 16 with the adjustment circuit assembly 18 and the reading circuitassembly 20. Fluid is allowed to flow through the valve 16 from an inletconnector 24 to an outlet connector 26 depending on a valve settingindicative of a cracking pressure (when valve 16 is embodied as a checkvalve) for valve 16. The adjustment circuit assembly 18 defines a numberof settings to alter pressure and/or flow characteristics of fluidthrough device 12. Adjustment circuit assembly 18 is coupled to valve 16through connector assembly 22 to alter the pressure setting based onsignals (e.g., an electromagnetic field) from tool 14. Reading circuit20 is also coupled to valve 16 through connector assembly 22 andconfigured to provide a signal indicative of the pressure setting totool 14 in response to a signal (e.g., an electromagnetic field) fromtool 14. Device 12 can be formed of biocompatible materials in order tobe subcutaneously positioned within a patient. Additionally, thematerials can limit the use of magnetic materials such that a pressuresetting for device 12 will not be altered during an MRI procedure.

Tool 14 includes a power source 30 configured to provide power to anadjustment interface 32, a reading interface 34 and a user interface 36.An exemplary tool is further described in co-pending U.S. patentapplication Ser. No. 13/015,195, filed on Jan. 27, 2011, entitled“Reading and Adjusting Tool for Hydrocephalus Shunt Valve”, the contentsof which are hereby incorporated by reference in their entirety.Adjustment interface 32 of tool 14 is adapted to provide signals (e.g.,an electromagnetic field) to adjustment circuit assembly 18 withindevice 12. In particular, the adjustment interface 32 can send signalsthat match a resonant frequency of the adjustment circuit assembly 18 inorder to induce a current therein. This current is used to adjust thepressure setting for valve 16. In one embodiment the resonant frequencyof adjustment circuit assembly 18 is approximately 100 kHz, althoughother frequencies can be used.

In a similar manner, reading interface 34 is adapted to send a signal(e.g., an electromagnetic field) to reading circuit assembly 20 thatmatches a resonant frequency of the reading circuit assembly 20.However, the resonant frequency of reading circuit assembly 20 changesas a function of the pressure setting for valve 16. As a result, readinginterface 34 is configured to transmit signals for multiple frequencies(e.g., by performing a scanning operation) and determine which frequencyis the resonant frequency for reading circuit assembly 20. Inparticular, when the frequency of the signal sent by reading interface34 matches the resonant frequency of reading circuit assembly 20,current will be induced within the reading circuit assembly 20, creatinga magnetic field that can be sensed by reading interface 34. In oneembodiment, the resonant frequency of reading circuit assembly 20 isaround 1 MHz, adjustable within a range of frequencies capable ofgeneration by reading interface 34. Using the resonant frequencyinformation, the pressure setting of valve 16 can be determined, forexample using a lookup table. User interface 36 can provide a visualindication of operation for adjustment interface 32 and readinginterface 34, allow input to the tool 14 and provide a visual indicationof proximity of the tool 14 to device 12. For example, user interface 36can include a screen to display pressure information, one or morebuttons to alter operation of tool 14 and/or a set of indicators.

FIG. 2 is a schematic view of adjustment circuit assembly 18 positionedwithin device 12, including a setting coil 40, an adjustment capacitor42, a resistive element 44 (herein embodied as a wire) and a crimpingstructure 48. As discussed above, in one embodiment, adjustment circuitassembly 18 is a series resonant circuit configured to resonate at aparticular frequency based on signals received from tool 14 (e.g., fromadjustment interface 32). Signals from tool 14 that match the resonantfrequency of adjustment circuit assembly 18 will create a current incoil 40, which will then pass through capacitor 42 and element 44.

As electricity flows through adjustment circuit assembly 18, element 44is heated resistively, resulting in a mechanical movement of the element44 acting against crimping structure 48. In particular, element 44 ismade of a suitable shape memory alloy (SMA), which is heated resistivelywhen a current is induced in coil 40. Although herein embodied as awire, element 44 can take other forms such as a flat stock. As can beappreciated, shape memory alloys return to a specific shape and/or sizethrough a temperature dependent phase change. Element 44 is configuredto contract upon reaching a transition temperature to a contracted stateand thus provide a mechanical movement for which to alter a valvesetting for valve 16. Upon cooling, element 44 then returns to anexpanded state. Crimping structure 48 holds element 44 in place at bothends of the element 44, which forms a loop opposite the crimpingstructure. In alternative embodiments, element 44 need not form a loopand can be coupled to connector assembly 22 in various differentmanners. In one embodiment, the element 44 is formed of nickel titanium(nitinol) and has a transition temperature of around 70 degrees Celsius.Upon reaching the transition temperature, element 44 contracts apredetermined length, which in one embodiment is around 2.5% of anoverall length of the element 44. When element 44 is coupled toconnector assembly 22, this contraction then triggers movement ofconnector assembly 22 in order to alter a pressure setting for the valve16. As illustrated in FIG. 2, element 44 forms a loop at an end oppositecrimping structure 48. This loop can be coupled to the connectorassembly 22 so as to translate mechanical movement thereto uponcontraction of the element 44. As contraction of element 44 (rather thanutilization of a magnetic mechanism) serves as the trigger to adjust thepressure setting for valve 16, inadvertent adjustment of the pressuresetting for valve 16 can be avoided during an MRI procedure.

FIG. 3 is a schematic diagram of reading circuit assembly 20 withindevice 12. Reading circuit assembly 20 includes an antenna coil 50, areading capacitor 52, a sensing coil 54 and a moveable member 56.Moveable member 56 is coupled to connector assembly 22 and, in oneembodiment, is connected in fixed relation thereto. Similar toadjustment circuit assembly 18, reading circuit assembly 20 can be aseries resonant circuit in which signals provided by reading interface34 of tool 14 are received by antenna coil 50. Signals received fromtool 14 that match the resonant frequency of circuit 20 will energizeantenna coil 50, causing electricity to be transmitted through capacitor52 and sensing coil 54. In an alternative embodiment, antenna coil 50can be eliminated such that a resonant frequency of capacitor 52 sensingcoil 54 is matched by tool 14. Movable member 56 can be formed of amaterial that changes the resonant frequency of circuit 20 as themovable member 56 moves relative to sensing coil 54. In one example,moveable member 56 is formed of a ferrite and is cylindrically shaped.The ferrite alters inductance of sensing coil 54, depending on how muchof the ferrite material is positioned within the sensing coil 54. Inanother embodiment, the movable member 56 can be “E” shaped such that amiddle portion is positioned within an interior of sensing coil 54 whileupper and lower arms of the member 56 are positioned outside sensingcoil 54. In any event, reading interface 34 senses a magnetic fieldcreated by electricity passing through circuit 20, which is indicativeof a pressure setting for device 12.

In alternative embodiments, adjustment circuit assembly 18 and readingcircuit assembly 20 can be modified in various configurations. Forexample, in one embodiment, setting coil 40 and antenna coil 50 can becombined into a single coil. The preferred or this combined circuit canfurther be implemented with diodes, if desired. In other embodiments,capacitors 42 and/or 52 can be eliminated.

FIGS. 4 and 5 illustrate a device 100 that is operable as ahydrocephalus flow control device (e.g., as device 12 in FIG. 1). Device100 includes a valve 102, an adjustment circuit assembly 104, a readingcircuit assembly 106 and a connector assembly 108. Details of thesecomponents of device 100 are provided below. In general, connectorassembly 108 is directly coupled to valve 102 in order to provide aparticular cracking pressure for device 100, as a function of a positionof connector assembly 108 relative to valve 102. In turn, adjustmentcircuit assembly 104 and reading circuit assembly 106 are coupled toconnector assembly 108 in order to adjust a position of connectorassembly 108 and transmit a signal indicative of a position of theconnector assembly 108, respectively.

In the illustrated embodiment, valve 102 is operable as an adjustablecheck valve and includes a valve seat 110 and a corresponding valvemember 112 moveable with respect to the valve seat 110. A valve spring114 urges valve member 112 against valve seat 110, creating a crackingpressure for valve 102. In general, valve spring 114 is coupled toconnector assembly 108 and alters pressure (i.e., force) placed on valvemember 112 as a function of the position of connector assembly 108. Asfluid pressure builds within valve 102, valve member 112 is urged awayfrom valve seat 110. When the fluid pressure is at a level to overcomethe pressure that valve spring 114 places on valve member 112 againstvalve seat 110, fluid will flow through valve 102. Valve 102 can takeother forms in alternative embodiments. For example, valve spring 114(herein illustrated as a helical compression spring) can be embodied asa flat spring wherein a position where force applied to the spring islaterally displaced from the valve member 112, either directly orthrough use of a fulcrum positioned between the valve member and theposition where force is applied to the spring. Although not illustratedherein, valve 102 can include other features as desired such as areservoir, siphon control device, etc.

Adjustment circuit assembly 104 is coupled to valve 102 throughconnector assembly 108 to alter a pressure setting of valve 102 andincludes a setting coil 120, an adjustment capacitor 122, a resistiveelement 124 and a crimping structure 126. Adjustment circuit assembly104 operates in a similar manner to adjustment circuit assembly 18discussed above. When positioned proximate device 100, tool 14 (FIG. 1)can operate to energize setting coil 120, thus inducing a currenttherein, which then passes through capacitor 122 and resistive element124. As current passes through element 124, the element 124 is heated,ultimately causing contraction of element 124 acting against crimpingstructure 126, which remains stationary as element 124 contracts. In oneembodiment, element 124 is insulated so as to prevent adjacentcomponents from overheating and/or becoming subject to electricalcurrent passing therethrough. As discussed below, element 124 is coupledto connector assembly 108 at an end opposite crimping structure 126 suchthat contraction of element 124 causes movement of connector assembly108 to adjust a pressure setting for device 100.

Reading circuit assembly 106 is configured to provide a signalindicative of the connector assembly 108 relative to valve 102 andincludes an antenna coil 130, a reading capacitor 132, a sensing coil134 and a moveable member 136. In the embodiment illustrated, moveablemember 136 includes a ferrite core 138 surrounded by a support member140, directly coupled to connector assembly 108. Reading circuitassembly 106 operates in a similar manner to reading circuit assembly 20discussed above. When positioned proximate device 100, tool 14 canoperate to energize antenna coil 130, thus inducing current therein,which then passes through capacitor 132 and sensing coil 134. Movablemember 136 and in particular ferrite core 138, moves to variouspositions with respect to sensing coil 134, which ultimately alters aresonant frequency for reading circuit assembly 106. Inducement ofcurrent in reading circuit assembly 106 generates a signal indicative ofthe pressure setting. The tool 14 can sense a magnetic field generatedby reading circuit assembly 106 to provide an indication for thepressure setting of device 100.

Connector assembly 108 includes a ratchet arm 150, a gear 152, a cam154, a cam follower 156, a push rod 160, a return spring 162 and aratchet spring 164. As discussed in more detail below, ratchet arm 150engages teeth of gear 152 to provide a rotational force thereto. Cam 154is rotationally fixed to gear 152 and includes teeth extending atvarying distances to engage cam follower 156. The teeth place varyingdisplacements on the cam follower 156, thus adjusting a position of camfollower 156 in relation to valve 102. As such, an amount of forceplaced on the valve member 112 relative to the valve seat 110 isadjusted. Depending on the position of cam follower 156, more or lesspressure is exerted on valve spring 114 by push rod 160. Return spring162 operates to bias cam follower 156 against cam 154. Additionally,ratchet spring 164 operates to reset ratchet arm 150 after providingrotational force to gear 152. In an alternative embodiment, ratchet arm150 can be replaced by a suitable escapement arm, including separateindex and drive pawls to rotate gear 152. In yet a further embodiment,ratchet arm 150 can be a linear ratchet movable in a linear direction(rather than rotationally) to adjust a pressure setting. Moreover, cam154 can be an axial cam that includes a plurality of steps with variableaxial displacements to engage a cam follower and adjust force placed onvalve spring 114.

Valve 102 is coupled to connector assembly 108 through push rod 160, andin particular push rod 160 is configured to push against valve spring114. Adjustment circuit assembly 104 is coupled to connector assembly108 through ratchet arm 150. As illustrated, element 124 of adjustmentcircuit assembly 104 is directly coupled to ratchet arm 150. Camfollower 156 of connector assembly 108 is directly coupled to moveablemember 136 of the reading circuit assembly 106. As such, movable member136 moves with cam follower 156 and with respect to sensing coil 134.

During adjustment of a pressure setting for device 100, setting coil 120is energized as discussed above such that electricity passes throughresistive element 124. When element 124 reaches a transition temperaturedue to resistance created therein, element 124 contracts, acting againstcrimping structure 126. At an end opposite crimping structure 126,element 124 is coupled to ratchet arm 150 through a connecting pin 170.When element 124 contracts, a force is applied to pin 170, causingratchet arm 150 to rotate about a pivot 174. In turn, rotation ofratchet arm 150 causes rotation of gear 152.

Gear 152 is rotationally fixed to cam 154, causing cam 154 to rotatewith gear 152. Cam 154 includes a variable radius about its rotationalaxis 176. In particular, cam 154 includes a plurality of teeth thatextend from rotational axis 176 at different lengths. In an alternativeembodiment, cam 154 can be smooth. Each tooth includes a peak (e.g.,peak 154 a), with valleys (e.g., valley 154 b) positioned on either sideof the peak. The plurality of teeth engage cam follower 156 and,depending on a rotational position of cam 154, pushes cam follower 156toward valve 102. In particular, contraction of element 124 to acontracted state causes cam 154 to rotate a predetermined distance suchthat cam follower 156, and in particular a cam follower peak 156 a,engages a peak of a corresponding tooth on cam 154. Once element 124cools and returns to its expanded state, cam follower 156 moves to avalley immediately adjacent the peak on cam 154. Thus, cam 154 undergoesa two-step rotation, one step when element 124 is heated and contracts(causing cam follower 156 to engage a tooth peak) and one step when wire124 cools and expands (causing cam follower 156 to engage a toothvalley). As such, element 124 indexes the valve 102.

As cam follower 156 is pushed toward valve 102, push rod 160 engagesvalve spring 114 and pushes valve member 112 against valve seat 110.Depending on a position for cam follower 156, more or less pressure ispressure is provided on valve member 112, ultimately controlling anamount of fluid that passes through valve 102. As cam follower 156 moveswith respect to valve 102, movable member 136 moves with respect tosensing coil 134. As movable member 136 moves with respect to sensingcoil 134, the resonant frequency for reading circuit assembly 106changes. As a result, the resonant of frequency is indicative of apressure setting for device 100.

FIGS. 6 and 7 illustrate an alternative embodiment of a flow controldevice 200. Device 200 operates in a similar manner to device 100 inFIGS. 4 and 5. Device 200 includes valve 202, an adjustment circuitassembly 204, a reading circuit assembly 206 and a connector assembly208. In general, these elements operate similar to correspondingelements as discussed with respect to FIGS. 4 and 5 above. For sake ofbrevity, several elements are not illustrated or discussed. Workersskilled in the art will appreciate that similar structure from device100 can be incorporated into device 200. In the embodiment illustrated,however, cam follower 256 is in-line with the movable member 236 andcorresponding ferrite core 238. Moreover, the gear 252 is located on topof cam 254 and a detent arm 280 is provided to engage teeth in the gear252.

Although the present disclosure has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges can be made in form and detail without departing from the spiritand scope of the present disclosure.

What is claimed is:
 1. An adjustable shunt system, comprising: animplantable flow control device, comprising: a valve seat; a valvemember interfacing with the seat to establish a pressure settingindicative of a pressure where fluid will flow through the valve seat;an adjustment circuit assembly including a resistive element coupled tothe valve member and operable to adjust force on the valve memberrelative to the valve seat so as to alter the pressure setting inresponse to a current applied to the resistive element; and a readingcircuit assembly including an antenna coil, a sensing coil and a membermovable with respect to the sensing coil, the member configured to altera resonant frequency of the reading circuit assembly as a function of aposition of the member with respect to the sensing coil; and a tool,comprising: an adjustment circuit operably couplable with the adjustmentcircuit assembly so as to generate an oscillating electromagnetic fieldso as to induce a current in the resistive element; and a readingcircuit operatively couplable with the reading circuit assembly andconfigured to induce a current in the antenna coil to generate a signalindicative of the pressure setting.
 2. The system of claim h wherein theadjustment circuit assembly further includes a setting coil electricallyconnected to the resistive element.
 3. The system of claim 2, whereinthe adjustment circuit assembly further includes a capacitorelectrically connected to the setting coil and resistive element.
 4. Thesystem of claim 1, wherein the reading circuit assembly further includesa capacitor electrically connected to the antenna coil and the sensingcoil.
 5. The system of claim 1, wherein the adjustment circuit isconfigured to generate signals at a first frequency and the readingcircuit is configured to generate signals at a second frequencydifferent than the first frequency.
 6. The system of claim 1, whereinthe reading circuit is configured to scan a signal across a range offrequencies in order to generate a signal in the antenna coil indicativeof the pressure setting.
 7. The system of claim 1, wherein theadjustment circuit generates a signal having a frequency ofapproximately 100 kilohertz.
 8. The system of claim 1, wherein thereading circuit generates a signal having a frequency of approximately 1megahertz.